Rap-GEF/Rap signaling restricts the formation of supernumerary spermathecae in Drosophila melanogaster

Tao and Rap2l ensure proper Misshapen activation and levels during Drosophila border cell migration

Collective cell migration is fundamental in development, wound healing, and metastasis. During Drosophila oogenesis, border cells (BCs) migrate collectively inside the egg chamber, controlled by the Ste20-like kinase Misshapen (Msn). Msn coordinates the restriction of protrusion formation and contractile forces within the cluster. Here, we demonstrate that Tao acts as an upstream activator of Msn in BCs. Depleting Tao significantly impedes BC migration, producing a phenotype similar to Msn loss of function. Furthermore, we show that the localization of Msn relies on its citron homology (CNH) domain, which interacts with the small GTPase Rap2l. Rap2l promotes the trafficking of Msn to the endolysosomal pathway. Depleting Rap2l elevates Msn levels by reducing its trafficking into late endosomes and increases overall contractility. These data suggest that Tao promotes Msn activation, while global Msn protein levels are controlled via Rap2l and the endolysosomal degradation pathway. Thus, two mechanisms ensure appropriate Msn levels and activation in BCs.

Dual regulation of Misshapen by Tao and Rap2l promotes collective cell migration

Collective cell migration occurs in various biological processes such as development, wound healing and metastasis. During Drosophila oogenesis, border cells (BC) form a cluster that migrates collectively inside the egg chamber. The Ste20-like kinase Misshapen (Msn) is a key regulator of BC migration coordinating the restriction of protrusion formation and contractile forces within the cluster. Here, we demonstrate that the kinase Tao acts as an upstream activator of Msn in BCs. Depletion of Tao significantly impedes BC migration and produces a phenotype similar to Msn loss-of-function. Furthermore, we show that the localization of Msn relies on its CNH domain, which interacts with the small GTPase Rap2l. Our findings indicate that Rap2l promotes the trafficking of Msn to the endolysosomal pathway. When Rap2l is depleted, the levels of Msn increase in the cytoplasm and at cell-cell junctions between BCs. Overall, our data suggest that Rap2l ensures that the levels of Msn are higher at the periphery of the cluster through the targeting of Msn to the degradative pathway. Together, we identified two distinct regulatory mechanisms that ensure the appropriate distribution and activation of Msn in BCs.

Growth rate mediates hidden developmental plasticity of female yellow dung fly reproductive morphology in response to environmental stressors

Understanding how environmental variation influences even cryptic traits is important to clarify the roles of selection and developmental constraints in past evolutionary divergence and to predict future adaptation under environmental change. Female yellow dung flies (Scathophaga stercoraria) typically have three sperm storage compartments (3S), but occasionally four (4S). More spermathecae are thought to be a female adaptation facilitating sperm sorting after mating, but the phenotype is very rare in nature. We manipulated the flies' developmental environment by food restriction, pesticides, and hot temperatures to investigate the nature and extent of developmental plasticity of this trait, and whether spermatheca expression correlates with measures of performance and developmental stability, as would be expected if 4S expression is a developmental aberration. The spermathecal polymorphism of yellow dung fly females is heritable, but also highly developmentally plastic, varying strongly with rearing conditions. 4S expression is tightly linked to growth rate, and weakly positively correlated with fluctuating asymmetry of wings and legs, suggesting that the production of a fourth spermatheca could be a nonadaptive developmental aberration. However, spermathecal plasticity is opposite in the closely related and ecologically similar Scathophaga suilla, demonstrating that overexpression of spermathecae under developmental stress is not universal. At the same time, we found overall mortality costs as well as benefits of 4S pheno‐ and genotypes (also affecting male siblings), suggesting that a life history trade‐off may potentially moderate 4S expression. We conclude that the release of cryptic genetic variation in spermatheca number in the face of strong environmental variation may expose hidden traits (here reproductive morphology) to natural selection (here under climate warming or food augmentation). Once exposed, hidden traits can potentially undergo rapid genetic assimilation, even in cases when trait changes are first triggered by random errors that destabilize developmental processes.

Female yellow dung flies naturally vary in number of sperm storage compartments (3S or 4S).

This spermathecal polymorphism is strongly heritable but also developmentally plastic.

4S expression is linked to growth rate and weakly correlated with fluctuating asymmetry, so potentially a developmental aberration.

There are mortality costs as well as benefits for 4S phenotypes, suggesting adaptive life‐history trade‐offs.

Spermathecal plasticity differs in the closely related and ecologically similar Scathophaga suilla.

Environmental changes can expose hidden traits with initially no function to natural selection.

Reproductive consequences of an extra long-term sperm storage organ

Background

Sperm storage plays a key role in the reproductive success of many sexually-reproducing organisms, and the capacity of long-term sperm storage varies across species. While there are theoretical explanations for why such variation exists, to date there are no controlled empirical tests of the reproductive consequences of additional long-term sperm storage. While Dipterans ancestrally have three long-term sperm organs, known as the spermathecae, Drosophila contain only two.

Results

We identified a candidate gene, which we call spermathreecae (sp3), in which a disruption cause the development of three functional spermathecae rather than the usual two in Drosophila. We used this disruption to test the reproductive consequences of having an additional long-term sperm storage organ. Compared to females with two spermathecae, females with three spermathecae store a greater total number of sperm and can produce offspring a greater length of time. However, they did not produce a greater total number of offspring.

Conclusions

Thus, additional long-term sperm storage in insects may increase female fitness through extending the range of conditions where she produces offspring, or through increasing the quality of offspring via enhanced local sperm competition at fertilization.

Rap1 GTPase promotes coordinated collective cell migration in vivo

During development and in cancer, cells often move together in small to large collectives. To move as a unit, cells within collectives need to stay coupled together and coordinate their motility. How cell collectives remain interconnected and migratory, especially when moving through in vivo environments, is not well understood. The genetically tractable border cell group undergoes a highly polarized and cohesive cluster-type migration in the Drosophila ovary. Here we report that the small GTPase Rap1, through activation by PDZ-GEF, regulates border cell collective migration. We find that Rap1 maintains cell contacts within the cluster, at least in part by promoting the organized distribution of E-cadherin at specific cell-cell junctions. Rap1 also restricts migratory protrusions to the front of the border cell cluster and promotes the extension of protrusions with normal dynamics. Further, Rap1 is required in the outer migratory border cells but not in the central nonmigratory polar cells. Such cell specificity correlates well with the spatial distribution of the inhibitory Rapgap1 protein, which is higher in polar cells than in border cells. We propose that precisely regulated Rap1 activity reinforces connections between cells and polarizes the cluster, thus facilitating the coordinated collective migration of border cells.

The Rap activator Gef26 regulates synaptic growth and neuronal survival via inhibition of BMP signaling

In Drosophila, precise regulation of BMP signaling is essential for normal synaptic growth at the larval neuromuscular junction (NMJ) and neuronal survival in the adult brain. However, the molecular mechanisms underlying fine-tuning of BMP signaling in neurons remain poorly understood. We show that loss of the Drosophila PDZ guanine nucleotide exchange factor Gef26 significantly increases synaptic growth at the NMJ and enhances BMP signaling in motor neurons. We further show that Gef26 functions upstream of Rap1 in motor neurons to restrain synaptic growth. Synaptic overgrowth in gef26 or rap1 mutants requires BMP signaling, indicating that Gef26 and Rap1 regulate synaptic growth via inhibition of BMP signaling. We also show that Gef26 is involved in the endocytic downregulation of surface expression of the BMP receptors thickveins (Tkv) and wishful thinking (Wit). Finally, we demonstrate that loss of Gef26 also induces progressive brain neurodegeneration through Rap1- and BMP signaling-dependent mechanisms. Taken together, these results suggest that the Gef26-Rap1 signaling pathway regulates both synaptic growth and neuronal survival by controlling BMP signaling.

Control of cell adhesion dynamics by Rap1 signaling

Individual cells in their particular environments adhere to the extracellular matrix (ECM) and their neighbours via integrin-containing and cadherin-containing complexes, respectively. The dynamics of these interactions regulate the formation and maintenance of complex tissues. An expanding body of evidence accentuates the role of the small Rap1 GTPase and its associated signaling network in many of these processes. In this review we will discuss more recently revealed roles of Rap1 signaling by primarily focusing on functions of the Rap1 effectors RIAM, KRIT-1/CCM1 and AF-6/Afadin in junctional regulation of the vascular system and in epithelial cells. Furthermore, we will describe novel findings on the Rap activator PDZ-GEF in the regulation of cell-cell adhesion between epithelial cells and within a stem cell niche.

Possible atavisms of genitalia in two species of earwig (Dermaptera), Proreus simulans (Chelisochidae) and Euborellia plebeja (Anisolabididae)

Male and female genitalia generally show a rapid evolutionary rate, which raises the problems related to homologization and the determination of the polarities of evolutionary changes. In earwigs (Dermaptera), multiple or branched female sperm-storage organs (spermathecae) have been reported for members of the Karschiellidae, Pygidicranidae, and Diplatyidae, collectively termed the "basal" Dermaptera. Whether the complicated spermathecae represent a plesiomorphy or an apomorphy has not been resolved. Here I report the occurrence of multiple or branched spermathecae in gamma-irradiated samples of two earwig species, Euborellia plebeja (Dohrn, 1863) (Anisolabididae) and Proreus simulans (Stål, 1860) (Chelisochidae), which belong to the "higher" Dermaptera (Apachyidae, Labiduridae, Anisolabididae, Spongiphoridae, Chelisochidae, and Forficulidae). Females belonging to the higher Dermaptera normally have a single-unbranched spermatheca. I discuss examples of possible atavisms in relation to the evolutionary pathways of spermathecal morphology. Possible atavisms in the number of male organs for sperm transfer (virgae) are also reported.

Quantitative Morphological Signatures Define Local Signaling Networks Regulating Cell Morphology

Although classical genetic and biochemical approaches have identified hundreds of proteins that function in the dynamic remodeling of cell shape in response to upstream signals, there is currently little systems-level understanding of the organization and composition of signaling networks that regulate cell morphology. We have developed quantitative morphological profiling methods to systematically investigate the role of individual genes in the regulation of cell morphology in a fast, robust, and cost-efficient manner. We analyzed a compendium of quantitative morphological signatures and described the existence of local signaling networks that act to regulate cell protrusion, adhesion, and tension.